The invention relates to a strain-gauge transducer comprising:
a strain gauge enclosed between pressure members, the strain gauge comprising a plate-shaped substrate provided with strain-detection elements, PA1 at least one spring arranged between said substrate and one of the pressure members so as to be deformed in conjunction with said substrate by pressures exerted by the pressure members.
Such a transducer is used for the detection or measurement of forces, in particular in force transducers. By means of these transducers it is possible to construct, for example, scales, load detectors etc.
In the analysis of the strength of materials it is known to place a sample on two supports and subsequently exert pressure on the other side of the sample in the middle of the two supports, use being made of its bending capacity. A similar device is utilised for measuring forces in transducers employing strain gauges formed by resistors arranged on a rigid substrate: two supports are fixedly connected to a first pressure member and the support arranged opposite thereto is fixedly connected to a second pressure member. This is described in, for example, "Strain sensitivity of thick-film resistors", J. S. SHAH, IEEE Trans. CHMT-3, no. 4, 1980, p. 554. The supports may be knife-edge supports. The knife-edge supports should be accurately parallel to one another, which requires accurate machining of the various elements.
A strain gauge generally comprises resistive elements whose resistance value varies with the deformation of the support on which they are arranged. Generally, several resistive elements are used which are connected in a Wheatstone bridge arrangement. In this way it is possible to obtain an electric signal which is directly proportional to the flexure of the support independently of temperature variations.
Thus, the document DE-B-1,001,832 describes a transducer element constructed as a symmetrical spring having two blades, of which one blade is provided with resistive elements, the deformation of the blade enabling pressure forces applied to the transducer element to be determined. The two blades are connected to one another in such a way that their deformations are equal and oppositely directed.
In order to ensure that the measurement by means of a transducer is linear and accurate the support which is used should have reproducible and linear mechanical properties. This is an advantage of gauges formed by a ceramic plate on which a resistive ink is deposited by screen-printing. Indeed, the ceramic plate has a very large elasticity range, which is limited only by the breaking point of the ceramic plate itself. There is neither plastic deformation nor hysteresis. For operation near the breaking point a means is required which prevents this point from being overstepped.
On the other hand, a strain-gauge transducer is mainly intended for use in consumer applications, for example in scales or all kinds of force transducers. For such consumer applications perfectly machined elements cannot be used for reasons of cost. This leads to the use of elements of imperfect flatness, parallelism and structure. The technologies used should therefore be free from the restraints associated with large series, in particular spreads of mechanical tolerances of the parts. It is to be noted that for the ceramic plates used the deflection to be utilised is small. Moreover, the ceramic plates for mass-production uses are commercially available plates, which are generally non-polished and have a surface roughness or a curvature which may be even of the order of magnitude of the deflection amplitude. The mechanical tolerances are not constant from plate to plate. An adjustment may be correct for one plate and may be destructive or lead to inadequate deflection amplitudes for another plate. In practice, it is therefore difficult to limit or adjust the maximum deflection that can be handled by such a plate so as to obtain a correct operation without breaking.